This invention relates to a hydraulic or pneumatic motor or pump (a “fluid-working machine”).
EP-B-0494236 introduces a mechanism for actuating electromagnetic poppet valves in a fluid-working machine, using the pre-compression and pre-expansion of the contents of a working chamber to create a motoring cycle of the machine. Working chambers of the machine are selected in real time so that fluid power is controllably converted to shaft power and vice versa, thus allowing a controllable bi-directional energy flow. The nature of poppet valves is that the fluid pressure acts over the seating area creating a large closing force. The motoring cycle of EP-B-0494236 relies on the energy from an already spinning shaft to allow the opening of the solenoid-operated poppet valve against the pressure in the high-pressure manifold. This requirement limits the use of this technique to conditions where the machine is already operating above a threshold shaft speed. For applications with the requirement of starting from zero speed, the motoring cycle must be initiated by different means.
U.S. Pat. No. 5,735,582 describes an electromagnetic valve assembly comprising a master valve and an auxiliary valve, the auxiliary valve serving to open the master valve against a high pressure. In order for the master valve to open, fluid must flow along a capillary between the master valve body and the bore until the pressure is equalised. This means that actuation of the valve assembly requires 200 to 300 ms which is too slow for use in a fluid-working machine.